AAT1271 Datasheet, PDF (13/19 Pages) Advanced Analogic Technologies – 1.5A Step-Up Current Regulator for Flash LEDs

SwitchReg^TM
PRODUCT DATASHEET
AAT1271
1.5A Step-Up Current Regulator for Flash LEDs
Flash/Safety Timer – Address 1
A timer function that enables the flash current sinks for
a programmed amount of time is incorporated in the
AAT1271. The on-time is programmed by loading the
Timing Register at Address 1 with a value from 1 to 16
and by choosing a value for the external timing capaci-
tor, C_T (see Table 3). When data is latched into Address
1, the data will be used at the next occurrence of an
asserted FLEN, it will then revert back to the default
value of Data 1, or 16/16. This feature eliminates the
need for an external, housekeeping baseband controller
to contain a safety delay routine. It also serves as a
protection feature to minimize thermal issues with the
Flash LEDs in the event an external controller’s flash
software routine experience hang-up or freeze. If no
write instruction is applied to Address 1, then the safety
timer will default to the maximum delay programmed
externally at C_T.
The Flash Time T can be calculated by the following
equation:
T = 7.98s/μF · C_T (μF)
Where T is in seconds and C_T is in μF.
For example, using a 47nF capacitor at C_T sets the flash
timeout to:
Flash Timeout = 7.98s/μF · 47nF = 375ms
The relationship between the flash safety timeout and
the capacitance of the timer capacitor is illustrated in
Figure 2.
1200
1000
800
600
400
200
0
0
20
40
60
80
100
120
140
CT Capacitor (nF)
Figure 2: Flash Safety Timeout
vs. Timer Capacitor.
Data
1*
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Ratio of MM Timeout
16/16
15/16
14/16
13/16
12/16
11/16
10/16
9/16
8/16
7/16
6/16
5/16
4/16
3/16
2/16
1/16
Table 3: Address 1, Flash Safety Timer
Programming; Maximum Value
Programmed by C_T.
Output Enable Control – Address 2
In the case where two LEDs are used, each output can
be enabled or disabled independently in movie mode as
shown in Table 4. To enable or disable either or both
FLOUTA and FLOUTB, a write instruction to Address 2 is
applied to the AAT1271’s EN/SET pin. If no write instruc-
tion is applied, the default value for Address 2 is FLOUTA,
FLOUTB = OFF. During a flash event, both FLOUTA and
FLOUTB will be enabled regardless of the movie-mode
setting.
Data
1*
2
3
4
Selection
FLOUTA
OFF
OFF**
ON
ON
FLOUTB
OFF
ON
OFF**
ON
Table 4: Address 2, Movie Mode
Output Configuration.
* Denotes the default value.
** A small current will flow due to the start-up test current.
1271.2008.08.1.1
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